Survival of the Sickest: Chapter 5 Summary & Analysis

For thousands of years, the Guinea worm has plagued humans. The larvae of the worm can be found in still water and ponds, and when people drink the water, the larvae enter their digestive system, grow over time, and mate. Once the parasites are fully grown, they make their way to the skin, secreting acid to burn an exit tunnel for themselves. The burning sensation causes humans to cool themselves with water, and when the worm senses this, it emits a milky fluid full of thousands of larvae and the process starts over.

Former president Jimmy Carter has led a two-decade effort to inform people worldwide about how the Guinea worm is contracted and how it reproduces. His efforts have helped educated humans about water that could be infected and counseled them not to use water to relieve the burning sensation. This has caused infections to drop from 3.5 million in 1986 to 10,674 in 2005. By understanding how the Guinea worm has evolved, we can protect people from it.

As humans evolve, infectious diseases evolve alongside us, and their “hardwired imperatives” are the same: survive and reproduce. Moalem emphasizes that not all bacteria and viruses are bad, however. About 1,000 different types of microbial creatures live in the human body, most of them in the digestive system. These gut flora help to create energy by breaking down food products for us, training our immune systems to identify and attack harmful organisms, and stimulating cell growth. This is why antibiotics can cause digestive issues, because they also kill helpful bacteria as well as harmful ones. The human body also frequently houses bacteria that can be harmful, but the gut flora can prevent those dangerous bacteria from growing to dangerous levels.

Not all relationships between organisms and their hosts are so symbiotic (mutually beneficial). The Guinea worm, for example, is a pure parasite. And when its victims feel the urge to seek out water, the infected person is experiencing “host manipulation,” which occurs when a parasite provokes its host to behave in a way that helps the parasite to survive and reproduce.

Moalem then examines other examples of host manipulation in nature. In Central America, there lives a kind of spider that spins orb-shaped webs. A particular species of wasp then stings the spider, paralyzing it, and lays eggs on its abdomen. The spider continues as normal, but when the egg hatches into a wasp larva, it makes holes in the spider’s abdomen and the larva feeds on its blood. When the larva is ready to cocoon, it injects the spider with chemicals, and the spider completely changes its behavior. Instead of building circular webs, it builds a special web to protect the larva’s cocoon. When it is finished, it sits in the center of the web, and the wasp kills the motionless spider and builds a cocoon inside the webs before hatching a week later.

Host manipulation generally involves an important step in the parasite’s reproduction. Adult flukes, a kind of worm which live in sheep, need to find a way to get into the body of another sheep so that the species doesn’t die out when the original host sheep dies. The eggs of the fluke are discarded with the sheep’s dung. A snail then eats the eggs, and when they hatch, they are secreted by the snail as slime. Ants feed on the slime, and the flukes inhabit the ant. Then, the fluke manipulates the ant’s brain: every night it causes the ant to climb to the tip of a blade of grass, waiting to be eaten by a passing sheep.

Viruses and bacteria also engage in host manipulation. For instance, the rabies virus colonizes the salivary glands of its host, making it difficult to swallow and causing foaming at the mouth. The virus also chemically induces the animal to be more aggressive and agitated, leading it to bite. When the anima’s mouths is foaming with rabies-filled saliva, its bites are infectious. In all of these cases, the host isn’t acting in a completely new way—rather, the parasite has evolved to influence its host to help the parasite  survive and reproduce. However, we can learn to shift the evolution of the parasite so that it’s less harmful to us.

T. gondii is a parasite that can infect any mammal but can only reproduce in cats. The spore cells of T. gondii, called oocytes, can last a year outside a host. They can infect an animal when they are eaten or when animals eat the flesh of an infected animal. Once an animal is infected, T. gondii cells insert themselves inside brain and muscle cells. T. gondii infects as much as half the world’s human population, but it is generally benign in humans. In mice, however, the parasite causes them to become fat and lethargic and lose their fear of predators—it even draws them to the scent of cat urine, which gets T. gondii back to cats.

Moalem examines another form of host manipulation: sneezing. Most people think of sneezes as symptoms. But when a person is already infected with the cold virus, it is actually a form of host manipulation: the cold virus learns to trigger the sneezing reflex in order to help spread the virus to other people.

Pinworms also use host manipulation. Pinworm infection is one of the most common infections contracted by children. Pinworms grow to maturity in the large intestine and then make their way out to deposit microscopic eggs on the skin. They also deposit allergens to cause itching. When children scratch their skin, eggs get under their fingernails, and the children deposit those eggs to anything they touch—including into their mouths, where the process starts again.

Cholera uses more passive methods. Cholera is a waterborne disease that causes severe diarrhea. Diarrhea is how the disease reenters the water supply and ensures its ability to find new hosts. Malaria also manipulates human hosts by incapacitating them—when someone with malaria is incapacitated, they are a helpless target for mosquitoes. Mosquitoes that bite infected humans then pick up those malaria-carrying protozoa and are able to infects others.

Diseases affect human behaviors in other ways, outside of host manipulation. This includes instinctual behavior: for example, disgust at sights and smells prompt us to avoid things that are full of harmful bacteria, like animal waste or spoiled food. It also includes learned behavior like covering one’s nose and mouth when sneezing or washing one’s hands before a meal.

There are additional behaviors that may have developed to put the survival of a species over the survival of individuals: for instance, sick primates, beetles, and birds wander away from their kin to protect them from infection. Additionally, some species like lobsters have evolved to avoid their brethren when they become infected (even before showing signs of infection). This may also be the biological reason for xenophobia, as we instinctually perceive outsiders as threats to our own health and survival—even if that instinct is no longer practical.

Just as we have been evolving to survive disease, organisms that cause disease have been evolving alongside us. Penicillin was first used to treat staph infection in 1942. Eight years later, 40 percent of staph infections were penicillin-resistant. We then developed methicillin to treat those strains in 1959. Two years later, the first methicillin-resistant staph (MRSA) was reported. Treatment then switched to use vancomycin, and the first case of vancomycin-resistant staph infection (VRSA) was reported in 1996.

Even if these organisms can evolve quickly, we can use their biology to direct their evolution to be less virulent (less harmful to their hosts). Paul Ewald, a pioneer of evolutionary biology, examined how microbes can move from one host to another. He found that they do so in three basic ways: close proximity through air or physical contact (like the cold or STDs); through an intermediate organism (like malaria); or through contaminated food or water (like cholera). Ewald asserted that diseases that travel through physical contact face pressure against virulence: they rely on hosts to carry them around, and therefore their hosts must be healthy enough to be mobile. But when infectious agents don’t need hosts to be mobile, they are often deadlier.

However, Ewald believes that understanding transmission methods can help us influence the evolution of parasites away from virulence. He takes cholera as an example: cholera can spread through humans or through water. If sewage flows easily into rivers that people wash in or drink from, then the cholera strain would evolve toward virulence because it can multiply freely without humans and rely on its access to the water supply for transmission. But if a country develops ways to protect its water supply, the bacteria should evolve to be less virulent because it will rely on humans to spread.

Ewald’s theory isn’t always applicable: some parasites are capable of surviving outside a host for a long time, like anthrax. But by understanding how organisms have evolved alongside us, we can potentially prevent parasites like the Guinea worm from spreading and reproducing or change the course of diseases like cholera and malaria.